Electrical geophones can be used to measure the velocity of a vibration by moving a coil of copper wire through a magnetic field based upon the vibration. This movement induces a voltage across the coil proportional to the movement which can be used to determine the velocity of the vibration. Analogously, a piezoelectrical ceramic or polyvinylideneflouride (PVDF) hydrophone sensor can create an electrical signal output that is proportional to sensed acoustic pressure. Traditionally, such sensors of the electrical type have required signal conditioning and preamplifying electronics near the sensing elements to be able to transmit the output signals to sensor array recording and processing equipment. These additional electronics can add significant complexity and cost to the outboard sensor suite.
The limitations of electrical sensor systems and improvements offered by a fiber optic system have been well documented. Further, the concept of using an optical fiber in sensing applications is not new. The U.S. Naval Research Laboratory (NRL) has been a leader in this area, and the NRL and others have disclosed a number of optical systems. For example, U.S. Pat. No. 4,648,083, issued to Gialorenzi, describes a typical fiber optic system. In this system, an optical phase equivalent to acoustic pressure in a hydrophone was measured. In addition, fiber optic vibration sensors have been disclosed by Hofler, Garrett and Brown of the Naval Post Graduate school. Common fiber optic sensors consist of coils of fiber wrapped around mandrels (see U.S. Pat. No. 4,525,818, issued to Cielo, et al.) or onto flexing disks (see U.S. Pat. No. 4,959,539, issued to Hofler, et al.). The coils are then attached to optical couplers to create an interferometer. In these conventional optical sensor systems, the physical phenomenon being measured is directly converted into a differential optical phase by acting on the interferometer. In other words, the acoustic pressures or vibrations stress the arms of the interferometer creating an optical phase shift in the interferometer. Some arrays require extended channel group lengths in order to achieve the required signal to noise ratio. In the case of a towed streamer array, a number of hydrophone elements (16 is common) are electrically connected together to create an output over an extended length. Optical versions of the extended group length have been described for example in U.S. Pat. No. 5,668,779, issued to Dandridge, et al. and U.S. Pat. No. 5,317,544, issued to Maas, et al. These extended interferometers are relatively complicated to fabricate and isolating only certain parts of the interferometer is difficult.
Another fiber optic sensor approach consists of fiber Bragg grating based sensors. The fiber Bragg gratings can be used in different manners to measure a given phenomenon. One method is to use the grating as a reflector, creating a Fabry-Perot interferometer. In this case, a similar change in phase of the light is measured. In a second method, the grating itself is the sensor, and strain on the grating changes the period of the grating which changes the wavelength of light reflected from the grating. This change in wavelength is proportional to the strain on the grating.
With either type of fiber optic sensor, sensor arrays can be significantly improved by the fiber optic telemetry. However, along the way, the sensors have become more complicated, and, in many cases, conventional fiber optic systems have yielded sensors with lower performance and/or higher cost.